Rollup merge of #66330 - Nadrieril:nonexhaustive-constructor, r=varkor

Improve non-exhaustiveness handling in usefulness checking

The comments around code paths for the `non_exhaustive` feature mention stuff like "we act as if the type had an extra unmatcheable constructor". So I thought I'd make this explicit by defining a special constructor that does exactly this.
This makes those code paths a bit more legible and less prone to error.